Fine structure restriction endonuclease maps of three classes of herpes simplex virus type I (HSV-1) defective DNA (dDNA) fragments cloned in lambda WES-gt B (ZO1-CP-04882-05-LMV and ZO1-CP-04914-02-LMV) have been determined. The major class, 9,000-10,000 base pairs, originates from the rightmost end of the HSV-1 genome. One of two minor classes of dDNA (6000-8000 b.p.) contains a site-specific 2000 b.p. deletion approximately 3000 b.p. from a region in dDNA representing the rightmost end in HSV-1. The other consists of at least two possible types of dDNA, those which contain 2000 b.p. less of HSV-1 of the rightmost DNA represented in the major dDNA class and a less well-understood type that appears to have internal rearrangements when compared to other members of this class and to the major dDNA class.